This invention relates to the field of thermal printing and, more particularly, to method and apparatus for preventing unevenness in printing depth in thermal printers.
Thermal printing apparatus is popularly used in facsimile receivers or other printers mainly because they are small, light, and easy to maintain. The thermal printing receivers used in facsimile apparatus and other printers have a thermal printing head which includes a line of heat generating elements, e.g., 2048 elements, and associated circuitry, i.e., registers and semiconductor switches. A line of data, each bit of which corresponds to each one of the elements, is supplied to the head along with a print enabling signal. Electric current flows through the heat generating elements to which print data is supplied when the print enabling signal is active. The elements then generate enough heat to cause a thermosensitive recording medium in contact therewith to darken. Lines of data are successively supplied to the head, along with print enabling signals, until the printing operation is completed.
Recently, there has been a need for thermal printing apparatus to print as rapidly as other printing apparatus. One method for increasing the printing speed is to reduce the driving cycle. However, it has been noted that by shortening the driving cycle unevenness in printing depth normally occurs. Because printing the current line begins at the time when heat due to printing the previous line remains, differences in temperature between previously heated elements and previously non-heated elements exists. Hence, the previously heated elements reach a higher temperature, or reach thermal saturation, sooner than previously non-heated elements. This results in unevenness in printing depth.
In order to prevent unevenness in thermal printing apparatus, it has been proposed to modulate the length or the magnitude of current to each element to be heated, in response to the presense or absence of previous printing, such that elements to be heated can generate the same magnitude of heat regardless of whether they were heated in the previous period. However, in order to do this, there is a need to provide the printing head with at least two lines of registers. Also, logic circuits are required for transforming the current data in accordance with the presence or absence of previous data. The logic circuits have to be provided so as to correspond to each bit. Moreover, when using the method of controlling the length of electric current, the printing head must be provided with a corresponding number of selecting gates for selecting one of several print enabling signals which have different active periods. When controlling the magnitude of electric current, two semiconductor switches are required for selecting either one of two current sources each having a different magnitude. Therefore, a large number of circuits must be integrated within the printing head. This would increase the cost of the printing head because the pass rate of manufactured printing heads decreases markedly in proportion to the degree of integration.